Atomic force microscopy (AFM) probes are often used to evaluate and measure features on a semiconductor or other product having very small dimensional or topographical features. Such products, which are typically formed on wafers, may also include magnetic read write heads such as those formed on a wafer of, for example, titanium carbide. AFM provides valuable metrology information during manufacture or development of such devices, in a manner that is substantially non-destructive to the device being manufactured. Conventional AFM probes typically include a silicon cantilever beam with a silicon tip (AFM tip extending perpendicular to or at a slight angle (e.g., few to 10 degrees) relative to the cantilever beam. The tip is often formed into a long and thin rod. The silicon tip is often etched to form a sharp apex small enough to fit into a deep feature. There are several high aspect ratio tips on the market made for imaging and measuring deep narrow features. Some high aspect ratio tips are made using a focused ion beam to machine the silicon tip into a long thin rod with an aspect ratio between 7:1 and 10:1. Hence, an AFM tip with a 10:1 aspect ratio (i.e., length:diameter) may be able to reach 1000 nm into a 100 nm diameter trench. Other high aspect ratio tips may be formed using electron beam deposition (e.g., EBBD tips) or may be carbon nanotubes with a diameter between 10 nm and 80 nm, for example.
Other AFM devices, such as Critical Dimension Atomic Force Microscopy (CDAFM), are designed for critical dimension information. This type of AFM device is equipped with a tip having a bulbous or expanded tip portion which is capable of profiling topographies having negative slopes (i.e., features with overhanging portions or negative sidewall profiles). With such a device, the bulbous portion of the AFM tip can extend under the overhanging portion of the topography. Profile chasing algorithms enabling tip to follow local topographical variation and algorithms for tip-profile deconvolution after scans are features in this AFM category for revealing critical metrology information of such overhanging or negatively sloped structure.
Although AFM provides a convenient, non-destructive means for determining a great deal of metrology or topographical information about a sample, the variety of information that can be provided by such a technique has been limited. For example, when a structure is constructed of multiple layers, AFM has not been able to determine the location of the interfaces between the different material layers. In addition, AFM scans, which provide a series of two dimensional profile plots have not been able to provide detailed critical dimension information in many circumstances. Such critical dimensions may include a maximum width location of a structure in three dimensions, or an exact transition point on a structure where the structure changes from one shape to another through local slope transition at the layer interface.
Typically, when such information has been needed, AFM techniques have had to be combined with other destructive testing techniques such as focused ion beam milling, wafer cleave experiments, or etching to expose the cross section. Understandably, such techniques add considerable time and expense to the manufacturing process.
Therefore, there is a strong felt need for a convenient, non-destructive testing technique that can provide material interfacial data and critical dimension information. Such a technique would preferably incur little added manufacturing expense, and would preferably employ existing testing techniques such as AFM, while still providing the necessary critical dimension and interfacial information.